The cross-compilers need to be outside the chroot. I tried to use
cross-compilers installed inside the chroot while building outside the chroot,
and they fail to link at runtime, since they expect their shared libraries to
be in /usr.

Note QMAKE_RPATHLINKDIR, which was not present in the
previous post:
since linking needs to happen against libraries in
/home/enrico/…/arm-linux-gnueabihf/…, we need to tell the linker not to go
and search in /.